Besides the known liquid fuels, such as petrol or diesel, gaseous fuels, in particular compressed natural gas (CNG), are being used increasingly in modern internal combustion engines for economic and environmental reasons. Since the natural gas filling station infrastructure is still not extensive, both liquid and gaseous fuels are often used for alternative operation of the internal combustion engine with both fuels. The term used here is bivalent operation of the internal combustion engine, as opposed to monovalent operation with just one single type of fuel.
The gas supply system of such an internal combustion engine usually comprises a gas reservoir, shut-off valves, temperature and pressure sensors, a pressure reducer or pressure regulator, a number of gas injection valves corresponding to the number of cylinders of the internal combustion engine, and corresponding fluid lines, together with an electronic control device.
The natural gas is usually stored in one or more cylinders at pressures of up to 200 bar. A pressure reducer or an electrical pressure regulator reduces this pressure to a lower value at the inlet to the gas injection valves. If the gas is injected into the inlet manifold of the internal combustion engine (port injection), typical pressure values lie in the range of 2-8 bar, depending on the characteristics of the injection valves.
In view of the advantages afforded by direct injection of the fuel into the combustion chamber of the internal combustion engine, such as lower emissions and reduced fuel consumption, systems are known which allow the natural gas to be injected directly into the combustion chambers of the internal combustion engine. In order to achieve this, however, the pressure of the natural gas on a rail or at the gas injection valve must be greater than in the case of inlet manifold injection of the natural gas. Typical values in this case lie in the range of 5-20 bar, in order to be able to introduce the necessary quantity of gas in a shorter available time span compared to inlet manifold injection.
The gas injection valves are usually embodied as so-called solenoid valves, in which a solenoid coil is provided as actuator and a nozzle needle as control element, these parts delivering the gas into a combustion chamber of the internal combustion engine or preventing the flow of gas depending on their electrical activation.
In the case of a gas injection valve the limits to the possible fuel flow rate are encountered very rapidly since, owing to the lower energy density of the gas compared to liquid fuels, for the same valve opening period a larger port cross section is needed than in the case of a liquid injection valve, in order to attain a specific power output of the internal combustion engine. The required port cross section can be achieved by an increased nozzle needle lift or a larger disk. A larger diameter of the valve disk has the disadvantage, however, that this increases the gas forces acting on the valve disk.
The increased lift means that the greater the distance between the solenoid coil and the control element that can be actuated thereby, the smaller the magnetic forces capable of raising the control element become. This means that the flow rate can exceed a specific amount for a preset electrical supply voltage and a preset supply current, and a given gas pressure.
For direct injection of the gas into the combustion chamber of an internal combustion engine, therefore, use is made of gas injection valves which have a valve needle opening outwards, that is to say into the combustion chamber. Besides a reduced flow resistance for the gas to be introduced, this has the further advantage that in addition to the force of the return spring the gas forces in the combustion chamber of the internal combustion engine also keep the valve port closed during combustion.
The problem with such gas injection valves is that the seat of the valve needle is exposed to the high combustion temperatures in the combustion chamber of the cylinder, therefore ruling out efficient sealing materials such as plastic or rubber as materials for the seat. It is thereby difficult to fulfill the prescribed leak-tightness requirements for components of CNG installations according to the ECE R 110 regulations. In addition, the high sealing forces necessary mean that the seat is subject to increased wear on closing of the valve. In order to minimize the wear, expensive paired material combinations are used for the seat and the valve disk. Such high-strength paired material combinations moreover tend to cause the valve needle to rebound.
DE 10 2009 012 688 B3 describes a valve for injecting gas which can be actuated by small magnetic forces. For this purpose a control element and a closing element are provided, the control element when in the closed position being situated at a short distance from the solenoid coil. The control element is operatively connected to the closing element in such a way that when an electrical current is passed through the solenoid coil the control element carries the closing element towards the solenoid coil. Furthermore, the closing element is attracted even closer to the solenoid coil and as far as an open position by magnetic forces of the solenoid coil, regardless of the movement of the control element. By providing the control element and the closing element it is possible to open an outlet port of the valve through a first movement of the control element and a linked movement of the closing element. This is accompanied by a fall in pressure in a delivery chamber, in which the control element and the closing element are situated. The gas pressure and the associated forces acting on the closing element are thereby reduced. A further movement of the closing element into an open position, in which the outlet port is fully opened, can therefore be achieved by small magnetic forces.
DE 10 2009 012 689 B4 discloses a valve for delivering gas into a combustion chamber of an internal combustion engine, the valve having a housing and an output chamber being provided in the housing. The delivery chamber has an outlet port, an actuator, in particular a solenoid valve having a coil and a magnetic closing element, being arranged in the housing. The closing element is assigned to the outlet port, the closing element opening or closing the outlet port depending on the actuation of the actuator. The closing element is guided in a guide of the housing, the closing element being guided through the delivery chamber to the outlet port and the closing element in a closed position in the delivery chamber being subjected to gas pressure only transversely to the moving direction of the closing element. The closing element can thereby be moved from the closed position into the open position with only slight counter-pressure, so that a less powerful actuator can be used for actuating the closing element.